The present invention concerns a process for recovering ethylene glycol formed in a method for the manufacture of ethylene oxide by the catalytic oxidation in the gaseous phase of ethylene with oxygen.
The two first steps customarily adopted in order to isolate the ethylene oxide from the gaseous mixture resulting from such a reaction are:
(1) An absorption consisting of placing the gaseous mixture in contact with water in order to form a dilute aqueous ethylene oxide solution containing gases in the dissolved state and especially carbon dioxide; and
(2) A desorption consisting of subjecting the aqueous ethylene oxide solution resulting from the absorption step to the action of steam in a column delivering at the bottom an aqueous stream practically devoid of ethylene oxide serving, after cooling, as absorption fluid in the first of the two steps just described.
This aqueous stream, designated in the description which follows by the expression "glycolated water", becomes progressively and continuously enriched in ethylene glycol formed by hydration of the ethylene oxide under the conditions to which this compound is subjected; particularly during the desorption stage, if a part were not evacuated from it in order to limit the concentration of ethylene glycol to a value generally not exceeding about 10% by weight.
The economics of the process for the manufacture of ethylene oxide, including pollution problems, demand that the ethylene glycol contained in the evacuated glycolated water be recovered.
In practice, amongst other things, glycolated water contains, besides ethylene glycol, other glycols such as diethylene glycol or triethylene glycol, troublesome impurities such as those pointed out in U.S. Pat. No. 3,904,656 and salts, principally sodium or potassium carbonates and sodium or potassium salts of carboxylic acids. The presence of these salts is due to the addition of anticorrosive sodium or potassium compounds, such as hydroxides, to the glycolated water which otherwise would produce corrosion phenomena, its ph being normally rendered acidic by the carbon dioxide and the carboxylic acids which it contains.
In order to separate the ethylene glycol from the stream of glycolated water evacuated from the cycle, the known processes recommend complex and expensive treatments.
Thus, U.S. Pat. No. 3,904,656 recommends the passage of the evacuated glycolated solution over ion exchange resins and over activated charcoal prior to directing the aqueous stream resulting from this treatment and containing the ethylene glycol toward a step of a process for the manufacture of ethylene glycol from ethylene oxide.
For its part, French Pat. No. 2,246,527 describes a treatment of evacuated glycolated water by inverse osmosis delivering on one hand an aqueous stream containing about 60% of the ethylene glycol initially contained in it and practically no longer any salts, and on the other hand an aqueous stream containing practically the entire salts and the complement of the ethylene glycol. The first one of the two aqueous streams is introduced into an installation for the production of glycol. The ethylene glycol contained in the second aqueous stream is not recovered.
According to French Pat. No. 2,295,938, the ethylene glycol is separated from the evacuated glycolated solution by placing the latter in contact with cation exchangers and anion exchangers prior to the separation of the glycols by distillation.
None of the known procedures realizes the separation of ethylene glycol in the concentrated form, directly from the evacuated glycolated water and without prior separation of the salts which it contains.